X0101 EFFECT 33 LIVE 2015-03-09 0001193125-15-076785 POS AM 0001397516 REX ENERGY CORP 333-196623 0001521777 Rex Energy I, LLC 333-196623-05 0001521781 Rex Energy Operating Corp. 333-196623-04 0001521859 Rex Energy IV, LLC 333-196623-03 0001521780 PennTex Resources Illinois, Inc. 333-196623-02 0001521778 RE Gas Development LLC 333-196623-01